An optical head integrating a light source and a photodetector has been proposed. The optical head disclosed in Japanese Patent Laid-Open 35737/1989 is an example. FIG. 13 shows a perspective view of the optical head disclosed above and FIG. 14 shows a schematic view of its optical system. The optical head chip 1a has first and second photodetectors 9 and 10 having five regions respectively on first and second substrates 2 and 8. The second substrate 8 is mounted on the first substrate 2. A semiconductor laser chip 5 is mounted on the second substrate 8 and a monitoring photodetector 4 is also formed on the second substrate 8.
A prism 11 is set on the first substrate 2. The prism 11 has a trapezoidal section, its grade plane 11a acts as a mirror and its vertical plane 11b faces to a exit of the semiconductor laser chip 5. A first diffraction grating 12 is formed on a top plane 11c of the prism 11. A second diffraction grating 13 is arranged above the first diffraction grating 12.
The outline of operation of this optical head is as follows: A beam emitted from the semiconductor laser chip 5 is reflected by the grade plane 11a of the prism 11 and incident onto the first diffraction grating 12. The diffraction grating 12 divides the beam into three beams, a zero-order beam for reading information and plus and minus first-order beams for detecting a tracking error. The three beams pass the second diffraction grating 13 and are focused by an objective lens 18 on a disk 19. The three beams focused on the disk 19 are reflected from the disk 19 and are incident onto the second diffraction grating 13 through the objective lens 18. The incident beams are divided into plus and minus first-order beams by the second diffraction grating 13 and transmitted to the first and second photodetectors 9 and 10.
Because the first and second photodetectors 9 and 10 are arranged in front of and behind the exact point of focus as shown in FIG. 15, spot sizes imaged on the photodetectors 9 and 10 associated with the shift of the focus point on the disk 19 cause the reverse change to each other. The focus error is detected by the beam size method utilizing this phenomenon. The tracking error is detected by a tri-beam method employing the plus and minus first-order beams for the tracking generated by the first diffraction grating 12.
When the semiconductor laser chip 5 is arranged in close proximity to the prism 11 to miniaturize further the integrated optical head mentioned above, it follows that either of the two first-order beams for detecting the tracking error reflected from the disk 19 passes the second diffraction grating 13 and is reflected from a surface faced to the second diffraction grating 13 on the semiconductor laser chip 5 and returns again onto the disk 19. The fact that the beam returned interferes with the two first-order beams for detecting the tracking error signal and the tracking error signal causes an offset as has been disclosed in Japanese Patent Laid-Open 24031/1986. For avoiding this interference, a method forming an optical shielding which scatters or absorbs the light at the exit of the semiconductor laser has been proposed in Japanese Patent Laid-Open 24031/1986.
However, in the miniature optical head integrating the light source and the photodetector as mentioned above, because either of the two first-order beams for detecting the tracking error reflected from the disk 19 is incident onto the surface faced to the second diffraction grating 13 on the semiconductor laser chip 5 or onto the surface located on the opposite side from said semiconductor laser chip 5 with respect to the grade plane 11a acted as the mirror and faced to the diffraction grating 13, the method proposed in Japanese Patent Laid-Open 24031/1986 can not prevent the interference.